417 research outputs found
Dressed tunneling approximation for electronic transport through molecular transistors
A theoretical approach for the non-equilibrium transport properties of
nanoscale systems coupled to metallic electrodes with strong electron-phonon
interactions is presented. It consists in a resummation of the dominant Feynman
diagrams from the perturbative expansion in the coupling to the leads. We show
that this scheme eliminates the main pathologies found in previous simple
analytical approaches for the polaronic regime. The results for the spectral
and transport properties are compared with those from several other approaches
for a wide range of parameters. The method can be formulated in a simple way to
obtain the full counting statistics. Results for the shot and thermal noise are
presented.Comment: 11 pages, 11 figures. Accepted for publication in Physical Review
Resonant tunneling through a small quantum dot coupled to superconducting leads
We address the problem of non-linear transport through discrete electronic
levels in a small quantum dot coupled to superconducting electrodes. In our
approach the low temperature I-V characteristics can be calculated including
all multiple quasi-particle and Andreev processes. The limit of very weak
coupling to the leads and large charging energies is briefly analyzed comparing
the calculated lineshapes of the I-V curves with recent experimental results.
When the coupling to the leads increases and Coulomb blockade effects can be
neglected, the combination of multiple Andreev processes and resonant
transmission gives rise to a rich subgap structure which largely differs from
the one found in the more studied S-N-S systems. We show how multiple processes
can be included within a simple sequential tunneling picture qualitatively
explaining the subgap structure. We suggest an experimental set-up where the
predicted effects could be observed.Comment: 11 pages, 4 postscript figures, to be published in Phys. Rev. B
(rapid communications
Subharmonic Gap Structure in Superconductor/Ferromagnet/Superconductor Junctions
The behavior of dc subgap current in magnetic quantum point contact is
discussed for the case of low-transparency junction with different tunnel
probabilities for spin-up () and spin-down ()
electrons. Due to the presence of Andreev bound states in the
system the positions of subgap electric current steps are split at temperature with respect to the
nonmagnetic result . It is found that under the condition
the spin current also manifests subgap
structure, but only for odd values of . The split steps corresponding to
in subgap electric and spin currents are analytically calculated and
the following steps are described qualitatively.Comment: 4 pages, 1 figure, minor stylistic changes, journal-ref adde
Interplay between Josephson effect and magnetic interactions in double quantum dots
We analyze the magnetic and transport properties of a double quantum dot
coupled to superconducting leads. In addition to the possible phase transition
to a state, already present in the single dot case, this system exhibits
a richer magnetic behavior due to the competition between Kondo and inter-dot
antiferromagnetic coupling. We obtain results for the Josephson current which
may help to understand recent experiments on superconductor-metallofullerene
dimer junctions. We show that in such a system the Josephson effect can be used
to control its magnetic configuration.Comment: 5 pages, 4 figure
Temperature dependence of Andreev spectra in a superconducting carbon nanotube quantum dot
Tunneling spectroscopy of a Nb coupled carbon nanotube quantum dot reveals
the formation of pairs of Andreev bound states (ABS) within the superconducting
gap. A weak replica of the lower ABS is found, which is generated by
quasi-particle tunnelling from the ABS to the Al tunnel probe. An inversion of
the ABS-dispersion is observed at elevated temperatures, which signals the
thermal occupation of the upper ABS. Our experimental findings are well
supported by model calculations based on the superconducting Anderson model.Comment: 6 pages, 7 figure
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